A motor produces torque ripple (torque pulsation) in principle, and thereby causes various problems such as vibration, noise, adverse influence on ride quality and electrical and mechanical resonances. Especially, in the case of an interior PM motor (IPMSM) spreading wide recently, there are produced cogging torque ripple and reluctance torque ripple compositely. As a countermeasure, various methods are under study and investigation, such as a countermeasure method of superposing a compensation current for cancelling torque ripple, on a torque command.
However, wariness or concern persists about adverse influence of analytical error, for example, in a method of performing a feedforward compensation by using a mathematical analysis model. Moreover, in a method of storing or memorizing a result of feedback learning control at a steady operating point, and performing feedforward compensation, online compensation is difficult because of time required for adjusting a control parameter adequately at each operation point. Furthermore, in a method of reducing current ripple, optimum suppression is not guaranteed in terms of torque ripple. Moreover, a torque ripple observer compensation method is under investigation. However, the torque ripple observer compensation method is still insufficient in validation for characteristic in speed varying operation, and online feedback suppression.
As to the above-mentioned problems, a feedback suppression control method by a shaft torque meter is already proposed, by the inventor of the present application and others (cf. a non-patent document 1, for example), in order to accurately suppress torque ripple which is a principal cause of electrical-mechanical resonance. In this control method, a control system is built to compensate for each of pulsation frequency components with attention being paid to the periodicity of torque ripples, and a function is provided for automatically adjusting a parameter so as to conform to or rapidly respond to operating condition change by using a result of system identification. This control method is explained below in detail.
(1) Basic Structure of Torque Ripple Suppressing Control Apparatus
FIG. 2 is a basic structure view of a torque ripple control apparatus of an earlier technology. The apparatus of this figure is applied to a system for driving a load with a motor.
A motor 1 becoming a source of torque ripple is connected with a load or load device 2 of some type, by a shaft 3. Its shaft torque is measured by a torque meter 4, and inputted into a torque ripple suppression device 5. Furthermore, information on a rotor position of the motor (phase) is inputted by using a rotation position sensor 6 such as a rotary encoder. Torque ripple suppression device 5 is provided with a torque pulsation suppressing means or device and configured to supply, to an inverter 7, a command or command value obtained by adding a toque pulsation compensating current to a command current produced in accordance with a torque command (or a speed command). In the example of FIG. 2, in consideration of current vector control performed by inverter 7, the torque ripple suppressing device 5 supplies command d-axis and q-axis currents id* and iq* or d-axis and q-axis current command values id* and iq* in a rotation coordinate system (orthogonal d and q axes) synchronous with the rotation of the motor.
It is known that the torque ripple (torque pulsation) is generated periodically in dependence on the rotor position, due to the structure of the motor. Therefore, the system uses a means or device for extracting torque ripple frequency component or components in synchronism with the motor rotation, and converts the torque ripple of an arbitrary order n into cosine and sine coefficients TAn and Tn [Nm]. Although there is a strict measuring means or device, such as Fourier transform, for measuring a torque ripple frequency component, the system can extract torque ripple frequency component by passage through a low-pass filter in a harmonic rotation coordinate system of a single phase using, as a reference, the rotational phase θ [rad] when weight is given to the ease in computation.
Torque ripple suppression device 5 performs the torque ripple suppression control using the above-mentioned cosine and sine coefficients TAn and TBn, and produces cosine/sine coefficients IAn, IBn [ampere] of a compensating current iqc* [ampere] of an arbitrary frequency component. Conversion into the compensating current iqc* is performed by using the same rotation phase θ at the time of conversion, according to a following equation (2). This compensating current is superposed on the q-axis current command and a normal vector control is performed.[Math 1]iqc*=IAn*·cos(nθ)+IBn*·sin(nθ)  (2)
FIG. 3 is a control block diagram of the torque ripple suppression device determining the cosine and sine coefficients TAn and TBn of the torque ripple from the above-mentioned detected shaft torque Tdet and the rotation phase θ, determining the compensating current cosine and sine coefficients IAn and IBn from these quantities and the rotational speed ω, and thereby determining the compensating current to suppress periodic torque disturbance. Symbols in the figure have following meanings.
T*: command torque or torque command value, Tdet: detected shaft torque or shaft torque detected value, TAn: nth order torque pulsation extracted component (cosine coefficient), TBn: nth order torque pulsation extracted component (sine coefficient), ω: detected rotational speed or number of revolutions, θ: detected rotational phase, iqc*: torque ripple compensating current, id: detected d-axis current, id*: command d-axis current, iq: detected q-axis current, iq*: command q-axis current, iu, iv, iw: u, v, w phase currents, iqo*: command q-axis current (before superposition of the compensating current), IAn: nth order compensating current cosine coefficient, IBn: nth order compensating current sine coefficient, and abz: rotation sensor signal. The letter n in the subscript represents an nth order torque ripple component.
In FIG. 3, a command converting section 11 converts the command torque T* into command d-axis and q-axis currents Id* and Iqo* in the rotation dq coordinate system in the vector control. In general, use is made of a conversion mathematical expression or table for achieving a greatest torque/current control. A current vector control section 12 functions to suppress torque pulsation by using, as command q-axis current iq*, a current obtained by superposition of torque pulsation compensating current iqc* on the command q-axis current Iqo. In the example of FIG. 3, the compensating current Iqc is superposed on command the q-axis current. However, it is optional to superpose the compensating current on the d-axis current or on both the d-axis and q-axis currents. Alternatively, in the system in which interference between the d-axis and q-axis currents is not problematical, it is possible to superpose a torque pulsation compensating signal directly on the command torque.
The current vector control section 12 performs operation of the current vector control in the d-axis and q-axis of the general orthogonal rotation coordinate system, and drives a load device 14 by driving a motor (IPMSM) 13 in the vector control mode. A coordinate transforming section 15 receives the three phase ac current iu, iv and iw detected by a current sensor 16, and the motor rotational phase θ, and produces, by conversion, currents id and iq of a d-axis and q-axis orthogonal rotation coordinate system synchronous with the motor rotation coordinates. A rotation phase/speed detector section 17 performs conversion from a rotation sensor signal abz of a rotational position sensor 18 such as an encoder, into information on the rotational speed ω and rotational phase θ.
A torque pulsation frequency component extracting section 19 extracts a periodic torque disturbance for each of pulsation frequency components, from the detected shaft torque Tdet detected by a shaft torque meter 20, and the rotational phase θ. Fourier transformation is a typical extracting means. Although a method for extracting pulsation component can be chosen arbitrarily, weight is given to ease in computation and an approximate Fourier transformation expressed by equations (3)-(5) is performed by multiplying the detected shaft torque Tdet [Nm], by nth order cosine wave and sine wave based on the rotation phase θ, and applying a low-pass filter to each. This is referred to as a torque ripple synchronous coordinate transformation.
                    [                  Math          ⁢                                          ⁢          2                ]                                                            {                                                                                                  T                                          dct                      ⁢                                                                                          ⁢                      A                                        n                                    ⁡                                      (                    t                    )                                                  =                                                                                                    T                        dct                                            ⁡                                              (                        t                        )                                                              ·                    cos                                    ⁢                                                                          ⁢                  n                  ⁢                                                                          ⁢                  θ                                                                                                                                              T                                          dct                      ⁢                                                                                          ⁢                      B                                        n                                    ⁡                                      (                    t                    )                                                  =                                                                                                    T                        dct                                            ⁡                                              (                        t                        )                                                              ·                    sin                                    ⁢                                                                          ⁢                  n                  ⁢                                                                          ⁢                  θ                                                                                        (        3        )                                {                                                                                                  T                                          dct                      ⁢                                                                                          ⁢                      A                                        n                                    ⁡                                      (                    s                    )                                                  =                                  ℒ                  ⁡                                      [                                                                  T                        dctA                        n                                            ⁡                                              (                        t                        )                                                              ]                                                                                                                                                                T                                          dct                      ⁢                                                                                          ⁢                      B                                        n                                    ⁡                                      (                    s                    )                                                  =                                  ℒ                  ⁡                                      [                                                                  T                        dctB                        n                                            ⁡                                              (                        t                        )                                                              ]                                                                                                          (        4        )                                {                                                                              T                  An                                =                                                      2                    ·                                                                  G                        F                                            ⁡                                              (                        s                        )                                                              ·                                                                  T                                                  det                          ⁢                                                                                                          ⁢                          An                                                                    ⁡                                              (                        s                        )                                                                              =                                      2                    ·                                                                  ω                        f                                                                    s                        +                                                  w                          f                                                                                      ·                                                                  T                                                  dct                          ⁢                                                                                                          ⁢                          An                                                                    ⁡                                              (                        s                        )                                                                                                                                                                                      T                  Bn                                =                                                      2                    ·                                                                  G                        F                                            ⁡                                              (                        s                        )                                                              ·                                                                  T                                                  det                          ⁢                                                                                                          ⁢                          Bn                                                                    ⁡                                              (                        s                        )                                                                              =                                      2                    ·                                                                  ω                        f                                                                    s                        +                                                  w                          f                                                                                      ·                                                                  T                                                  dct                          ⁢                                                                                                          ⁢                          Bn                                                                    ⁡                                              (                        s                        )                                                                                                                                                    (        5        )            
: Laplace transform, GF; pulsation extracting filter, ωf: pulsation extracting low-pass filter cutoff frequency [rad/s], s: Laplace operator.
A torque ripple suppression control section 21 performs a torque ripple suppression control to a component of each order by using TAn and TBn extracted and converted according to the equations (3)˜(5), and generates a cosine coefficient IAn* [A] and a sine coefficient IBn* [A] of an nth frequency component Iqcn* [A] of a compensating current iqc* [A]. Conversion to the nth compensating current Iqcn* is achieved by calculation using the same rotational phase θ as the rotation phase at the time of the torque ripple synchronous coordinate transformation, as the equation (2).
The command torque T* [Nm] is converted into the command d-axis and q-axis currents id* and iq0* achieving the greatest torque/current control, and the vector control is performed by superposing a resultant value iqc* of the compensating current of each order generated according to the equation (2), on command q-axis current iq0*. Basically, calculating operations performed in the torque ripple suppression control device are shaft torque pulsation component extraction, torque ripple suppression, and compensating current signal generation. Other operations are performed in a general inverter.
As typical form of the torque ripple suppression control section 21, it is possible to employ a periodic disturbance observer compensating method or a compensating current Fourier coefficient learning control method.
A compensating current generating section 22 generates the command compensating current iqc* according to the equation (2), and superposes the command compensating current iqc* on the command q-axis current.
(2) System Identification
The structure of the system shown in FIG. 2 forms a multi inertial axes torsional resonance system, with moments of inertia of PM motor 1, load device 2, torque meter 4, and coupling 3, etc. When the sensed shaft torque is fed back, a suppression control parameter must be determined adequately in accordance with operating condition because of the existence of a plurality of resonance•anti-resonance frequencies. Since a long learning time of the control parameter has a possibility of increasing the phenomena of electric and mechanical resonances, a rapid automatic adjusting function is required.
Therefore, in order to introduce a variable nominal control parameter adaptable to rotational speed change, the system disclosed in the non-patent document 1 identifies a system transfer function from the output to the input of the torque ripple suppression device 5 of FIG. 2, that is a frequency transfer function from the command compensating current iqc* to the detected torque Tdet of the shaft torque detector 4 in FIG. 3. Although it is possible to choose the system identifying method arbitrarily, FIG. 4 shows the result of non-parametric estimation of the frequency transfer function from a ratio of power spectrum densities of input and output by measuring the detected shaft torque Tdet with a computation cycle of 100 μs for 20 sec when Gaussian noise signal is provided to iqc* in a closed loop (a characteristic of an actual machine including the mechanical system, inverter current response, torque meter response, dead time, etc.). FIG. 4 further shows the result of parametric identification in the case of approximation with a four inertia system from tendency of the frequency transfer function. Although there are various optimization methods for approximation, employed is a method of evaluating error of amplitude characteristic up to 1 kHz in a frequency region, and performing a constrained nonlinear minimization (sequential quadratic programming method).
The system shown in FIG. 3 extracts only an arbitrary frequency transfer function from the result of identification of FIG. 4, in order to build a control system with coordinates synchronous with the torque ripple frequency. In the steady state, the amplitude and phase transfer functions of a system synchronizing with the torque ripple frequency can be expressed by a one-dimensional complex vector. Therefore, a system characteristic Psys in the control system of FIG. 3 is defined by a following equation (6).[Math 3]PSYS=PAm+PBm·i  (6)
PAm: real part of the system characteristic, PBm: imaginary part of the system characteristic, m: a frequency element number in a system identification table.
When, for example, the system characteristic in the range of 1˜1000 [Hz] is expressed by equation (6) for each of intervals of 1 Hz, it is possible to form a system identification table of 1000 elements of complex vectors. Use in the control system is always limited to only one complex vector. In accordance with a rotation speed change (a change in the torque ripple frequency), the system reads PAm and PBm instantaneously from the identification table, and applies an identification result obtained by conversion to a complex vector by linear interpolation, to the suppression control. To define the axes of the rear part and imaginary part based on the rotational phase, the cosine coefficient in equation (5) corresponds to the real part component, and the sine coefficient corresponds to the imaginary part component.
(3) Compensating Current Fourier Coefficient Learning Control Method
This is a torque ripple suppression control method explained as a method 1 in the non-patent document 1. In this method, a Fourier coefficient of a torque ripple frequency component is determined, and, from this, the compensating current iqc* is determined by calculation of the equation (2). This control method expresses the system transfer function of a frequency component synchronous with the torque ripple frequency in the form of a one dimensional complex vector, and extracts the real and imaginary parts of the torque ripple of an arbitrary frequency component by Fourier transformation etc. A feedback suppression control system is formed by applying the cosine and sin Fourier coefficients to the real and imaginary parts of the complex vector.
The compensating current coefficient is determined by an I-P (proportional•integral) learning control method. Proportional•integral gain is determined so that a closed loop characteristic of the I-P suppression control system matches a pole assignment of an arbitrary standard system reference model by model matching method. Moreover, these quantities act to adapt a parameter automatically by the use of the result of the system identification and the rotational speed information, and hence facilitate the implementation to a multi inertia resonance system.
At an arbitrary steady operating point (steady torque, steady rotational speed), the amplitude and phase of the compensating current at the time of completion of the suppression are stored. This operation is performed at a plurality of operating points, and thereby implementation is achieved in the form of a two dimensional table of the torque and the rotational speed. In this case, it is possible to input the torque and rotational speed information into the table, to generate the compensating current from the amplitude and phase data of the compensating current read from the table, and thereby perform a feed forward suppression.
(4) Periodic Disturbance Observer Compensating Method
This method is a torque ripple suppression control method recited as a method 2 in the non-patent document 1. The control parameter automatic adjusting process in the above-mentioned compensating current Fourier coefficient learning control method becomes lower in quickness of the response to a variable speed operation because disturbance is suppressed through adjustment of I-P control gain. Therefore, it is recommended to arrange the result of learning in the form of a table, and to use the table in feed forward suppression.
By contrast, the periodic disturbance observer compensating method estimates the torque ripple disturbance directly by using the idea of a periodic disturbance observer. Therefore, this method provides improvement in the problem of the response quickness. Accordingly, this method makes it possible to suppress torque ripple in an always online feedback mode even for a system having a varying speed and load variation. Moreover, the method facilitate implementation to the multi inertia resonance system by providing a function of automatically adjusting an inverse model of the periodic disturbance observer by using the result of system identification expressing with a one-dimensional complex vector like the compensating current Fourier coefficient learning control method.
FIG. 5 is a calculation block diagram of the torque ripple suppression control by the periodic disturbance observer. FIG. 5 shows a periodic disturbance observer section 31, a torque ripple extracting section 32 and an actual system 33. In this figure, dIAn*: command nth order periodic disturbance current real part (cosine coefficient) (command value), dIBn*: command nth order periodic disturbance current imaginary part (sine coefficient) (or command value), iqcn: nth order compensating current, dIAn: estimated nth order periodic disturbance real part (cosine) component (estimated value), dIBn: estimated nth order periodic disturbance imaginary part (sine) component (estimated value), IAn: nth order compensating current real part (cosine coefficient), and IBn: nth order compensating current imaginary part (sine coefficient).
Since FIG. 5 shows only the control system synchronous with the torque ripple frequency, the transfer function of the actual system is represented by a one-dimensional complex vector. That is, the actual system is represented by a following equation (7), and an equation (8) is the result of the system identification. Although the symbol “^” is added on top of a base letter P in estimated quantities in the equation (8) and subsequent equations, this symbol is expressed as “P^” in the description.[Math 4]Psys=PAn+PBn·i  (7){circumflex over (P)}sys={circumflex over (P)}An+{circumflex over (P)}Bn·i  (8)
PAn: system real part of the nth order torque ripple frequency component, PBn: system imaginary part of the nth order torque ripple frequency component, P^An: estimated system real part of the nth order torque ripple frequency component, and P^Bn: estimated system imaginary part of the nth order torque ripple frequency component.
A low pass filter transfer function of the torque ripple extracting section shown in FIG. 5 is given by a following equation (9), from the equation (5).
                    [                  Math          ⁢                                          ⁢          5                ]                                                                      G          F                =                  1                                                    (                                  1                  /                  wf                                )                            ⁢              s                        +            1                                              (        9        )            
The system of FIG. 5 has a structure identical to a general disturbance observer of an earlier technology. However, attention is focused only on the periodic disturbance. Therefore, the system characteristic is expressed by a one-dimensional complex vector as in equation (7), and the inverse system P^sys−1 of the observer section can be expressed simply by the inverse of equation (8) by using the result of the system identification.
                    [                  Math          ⁢                                          ⁢          6                ]                                                                                  P            ^                    SYS                      -            1                          =                  1                      (                                                            P                  ^                                An                            +                                                                    P                    ^                                    Bn                                ·                ⅈ                                      )                                              (        11        )            
After the complex vector operation, the periodic disturbance observer estimates the real part component and imaginary part component of the periodic disturbance (torque ripple) through observer filter, respectively. The estimated periodic disturbance real part dIAn and estimated periodic disturbance imaginary part dIBn are fed as compensating current so as to cancel the disturbance component as shown in FIG. 5. Normally, it is possible to suppress disturbance current of the frequency component by reducing the command periodic disturbance currents IAn* and IBn* to zero (0).